1. Technical Field
An embodiment of the invention relates generally to integrated circuit processing equipment, and in particular relates to the attachment of pellicles used in integrated circuit processing.
2. Description of the Related Art
Patterned reticles are used during the manufacture of integrated circuits to expose the surface layer (i.e., the top layer at that stage of manufacturing) of the wafer to a precision pattern of electromagnetic radiation (e.g., ultraviolet light of a particular wavelength). The exposure pattern at the surface layer may require tolerances of a few nanometers or less. Although the reticle pattern is typically much larger than the exposure pattern (the pattern is optically reduced before being focused on the wafer), particles of dust and other tiny bits of contamination on the reticle may cause fatal defects in the exposure pattern at the surface layer. Pellicles are used to prevent this problem by keeping the reticle pattern clean. A pellicle is a layer of effectively transparent material, mounted on a frame, that covers the patterned side of the reticle and seals off that side from contamination. Although the dust may still collect on the outside surface of the pellicle, it is far enough removed from the reticle pattern to be out of focus during the exposure operation, and therefore does not cause image defects. However, since the light must by necessity travel through the pellicle, the pellicle itself must have a minimal effect on the direction of travel of the light. Distortion of the pellicle from its desired flat shape may cause the direction of the light traveling through the pellicle to be affected, and thus affect the quality of the focused image.
Conventional pellicles are made of polymer film attached to a frame with adhesive, and the frame is then attached to the reticle, also with adhesive. The mechanical forces involved in this adhesive attachment process may be as high as 27,000 kilograms per square meter (kg/m2), which may cause the pellicle to be distorted from its desired shape (which would ideally be completely flat). The thinness of conventional polymer films (typically <1 micron thick) minimizes any resultant distortion in the image. However, the use of shorter wavelength lithography, such as 157 nm, has brought about the use of thicker, rigid pellicles made of materials such as fused silica. These rigid pellicles may be from 300 to 1,000 microns thick, and the effects on the focused image of distortion from the pellicle may be significant. The mechanical forces of adhesive attachment can introduce unacceptable distortion in the pellicle, thereby degrading the focused image and making the final image quality unacceptable.